Several methods and technologies have been developed for measuring the contents of large silos or bins used to store bulk materials such as minerals, coal, plastics, chemicals, and grains. One conventional method for measuring the amount of goods stored in large silos involves weighing the contents of the vessels. However, this method becomes difficult and impractical when the contents are stored in large, tall silos or other similarly large vessels.
Another conventional approach for measuring the amount of goods stored in large silos involves bolting strain sensors to the support structure of the silos. The contents of a vessel may be determined by monitoring the electrical output that is the direct result of strain measurements taken from one or more strain sensors coupled to the support structure of the vessel. By measuring the amount of axial strain in the support structure as material is added or removed from the vessel, the weight of the contents can be determined.
Conventional bolt-on strain sensors can be adversely affected by variations in the temperature of the support structure and/or the sensor itself. Even if a strain sensor is stable over a large range of temperatures that is commonly experienced if used outside, the strain sensor can still be susceptible to differences in temperature between the strain sensor and the support structure to which it is mounted. Fast temperature changes that can occur under variable solar irradiation can cause considerable measurement errors due to the temperature difference between the strain sensor and the support structure. For example, installing a single axis strain sensor on a silo support structure so that the strain sensor is sensitive in the direction of the principal strain will provide information about the weight or level of material in the silo. This information will be in the form of an electrical output proportional to the stress change in the support structure. However, the sensor will also measure and provide an electrical output of the strain related to thermal changes, i.e., the expansion/contraction of the support structure independent of the principal strain associated with a load change. Thus, the accuracy of bolt-on sensors can be affected by strains (expansion and contraction) induced by changes in the temperature of the support structure to which the sensor is bolted. The strain sensor is unable to discriminate between strains caused by loading or unloading of the support structure and thermal strains caused by changes in temperature.
One conventional approach for compensating for the measurement errors caused by temperature is to install a pair of bolt-on strain sensors on a support structure in a rosette pattern with the sensors arranged at right angles to each other. The electrical output of the vertical sensor is reversed as compared to the horizontal sensor so that, for example, the electrical output of the vertical sensor is positive for compression and negative for tension while the electrical output of the horizontal sensor is negative for compression and positive for tension. The vertical sensor is aligned with the principal strain so that the vertical sensor will be compressed when the load is increased. The horizontal sensor is at ninety degrees so that it will be tensioned in accordance with Poisson's ratio when the load is increased. The sensors are connected to each other so that the electrical output of the horizontal sensor is subtracted from the electrical output of the vertical sensor. For example, as a result of the sensors' orientation and electrical connections, increasing loads cause a voltage from the compressed vertical sensor to increase that is subtracted from the decreasing voltage generated by the tensioned horizontal sensor. Conversely, decreasing loads cause the voltage from the tensioned vertical sensor to decrease that is subtracted from the increased voltage generated by the compressed horizontal sensor. However, the vertical and horizontal sensors react equally to thermally induced strain. As a result, a decreasing temperature will compress both sensors causing the vertical sensor to generate an increasing voltage that is subtracted from the increasing voltage from the horizontal sensor. As a result, the combined voltage remains constant. Conversely, an increasing temperature will tension both sensors causing the vertical sensor to generate a decreasing voltage that is subtracted from the decreasing voltage of the horizontal sensor. Again, the combined voltage remains constant. Therefore, the rosette arrangement reduces measurement error caused by thermal effects.
One problem with using a pair of bolt-on strain sensors in a rosette pattern is that the sensors should be installed at precisely ninety degrees in relation to each other and as close to each other as possible so that they are both exposed to the same thermal changes. However, installing bolt-on strain sensors with the degree of precision required can be difficult. For example, installation requires drilling multiple holes in the support structure for fastening the strain sensor to the support structure, which may introduce alignment error.
Another conventional bolt-on strain sensor that compensates for measurement error caused by temperature and overcomes some of the problems with using individual bolt-on strain sensors in rosette pattern is disclosed in U.S. Pat. No. 5,734,110 entitled “Temperature Compensated, Easily Installed Bolt-on Strain Sensor.” The bolt-on strain sensor includes a sensor body of generally L-shaped configuration having mounting holes formed at the end of each leg of the L-shaped body and a common mounting hole formed at the intersection between the legs. The strain sensor is attached to the support structure using fastening elements that are inserted through each of the mounting holes. A first strain gage is mounted between two of the mounting holes of one of the legs while a second strain gage is mounted between the mounting holes of the other leg. The first strain sensor has an axis of sensitivity extending between the mounting holes of the first leg while the second strain sensing element has an axis of sensitivity extending between the mounting holes of the second leg. This conventional bolt-on strain sensor design reduces problems with misalignment associated with using a pair of individual strain sensors configured in a rosette pattern because the angle between the axes of sensitivity is fixed by the L-shaped configuration of the sensor body. However, using multiple fastening elements to secure the strain sensor to a support structure can produce other measurement errors. For example, a small shift in contact area caused by uneven tightening of the fastening elements can cause significant zero shifts that may exceed one hundred percent.
Therefore, it would be desirable for a strain sensor to be able to compensate for thermal expansion of the support structure that the strain sensor is mounted on. It would also be desirable that such a strain sensor be relatively simple to attach to the support structure so that additional measurement errors are not caused by installation.